The fact that Eddie Merckx smoked like a chimney immediately made some sense to me. A couple taps of the fingers found this... Effect of transdermal nicotine administration on exercise endurance in men. The short of it is that it wasn't smoking, it was nicotine, that was in part responsible for Merckx' legendary endurance. The positive effects of nicotine on endurance performance are probably very significant.
How addictive is nicotine when administered transdermally, buccally or orally? Probably, not very. Is nicotine a banned substance in professional cycling?